The population=<number> key can be used to indicate a rough number of citizens in a given place.

Whenever possible, it is recommended to also add the key source:population=* to the same object, having as it's value the name of the institution that counted the population and the year when the population was counted.
This way other people can easily verify and update these values. Also, for a more accurate designation of the population counting date, you can add a key population:date=* in ISO 8601 format.